In a direct-drive wind turbine, the rotor is usually realised as an outside rotor, i.e. the rotor is arranged on the outside and the stator is arranged on the inside so that the rotor encloses the STATOR. Here, the “rotor” is to be understood as the rotating component of the generator. The rotor is directly connected to the hub, so that when the hub rotates, the rotor rotates at the same rate. Usually, the rotor is also the field of the generator and the stator is the armature, i.e. the rotor usually bears the magnets while the stator bears the coils or windings. A housing of the rotating rotor can interface with a stationary canopy of the wind turbine by means of a labyrinth seal, so that the rotor is free to rotate, while an interior region of the canopy is sealed from the outside.
To obtain a high efficiency in a generator of such a direct-drive wind turbine, permanent magnets may be used. During operation of the generator, the stator windings become very hot, and heat convection and radiation given off by the windings acts to heat the magnets. At high temperatures, a permanent magnet becomes demagnetized. Therefore, some generator designs are based on the use of sintered permanent magnets made to include a quantity of Dysprosium, which ensures that the magnet does not lose its magnetization at high temperatures. However, such magnets are significantly more expensive. Therefore, much effort is invested in cooling arrangements with the intention of protecting the more economical permanent magnets from high temperatures.
In outside stator generator designs, the stator is on the outside and can give off heat to the ambient, air while the magnets of the inside rotor can be directly cooled by an inner cooling circuit of the wind turbine. Many prior wind turbines use some kind of cooling arrangement that involves a cooling airflow directed over the heat-generating parts. A forced cooling is more efficient, and such cooling arrangements may involve several fans for blowing and/or drawing air over critical parts in the interior of the generator. However, the cooling effect of such cooling arrangements is usually limited to an interior region.
In a direct-drive wind turbine with outer rotor, the magnets cannot be accessed by such an inner cooling circuit. Prior art direct-drive outer rotor wind turbines therefore rely on convection cooling to cool the rotor as it rotates through the surrounding air. However, the cooling effect is limited. A direct cooling of the magnets is made difficult since these are mounted in the outside rotor, and access to the magnet surfaces is limited to a very narrow air-gap between the magnets and the windings on the inside stator. Therefore, prior art cooling arrangements are characterised by a poor performance with regard to their ability to cool the permanent magnets in the outside rotor of a direct-drive wind turbine. Furthermore, in an air-cooled system, the stator windings give off heat to an airflow directed over the windings. This heated air in turn gives off heat to the magnets, which are already heated by the heat radiation given off by the windings, thereby worsening the problem.